Modest amounts of cryogenic liquid product can be produced from an air separation plant by boosting a portion of the air stream from the main air compressor, cooling it, then expanding it through a lower column turbine. For an internal compression cycle, efficient, cost effective turndown of the liquid production from the design point cannot be achieved with conventional cycles and/or turbomachinery. A solution is needed to enable a plant that is designed for high liquid production to decrease its liquid product with an associated power savings. Also, a plant that is to be built in a developing market can be designed for the eventual high liquid production rate, but can run initially at an efficient, lower production rate until the market grows.
The problem stems from the nature of a pumped liquid oxygen cycle, specifically with regards to the product boiler compressor. A portion of the air stream from the main air compressor is compressed, cooled, then condensed in a product boiler to vaporize the high pressure liquid oxygen stream. At each plant, the delivery pressure of the gaseous oxygen stream is fixed. While this pressure can vary from 50 to 500 plus pounds per square inch gauge, it remains constant at each plant. This requires that the compressor used to supply the high pressure feed air, referred to as the product boiler compressor, must discharge at a constant pressure. It is this fixed discharge pressure requirement that limits the variability in liquid product. Once a centrifugal compressor is designed and operated for a given discharge pressure and flow, a reduction in the suction pressure is not possible. Any reduction in suction pressure results in a corresponding decrease in outlet pressure, which means that the gaseous oxygen pressure requirement of the plant would not be met.
While the gaseous oxygen pressure at a given plant must be held constant, it is desirable to be able to vary the liquid production from the plant. The boosting of the air stream for liquid production is accomplished by either a separate compressor or by a booster loaded by the work output of the turbine. A reduction in liquid product from the design point is achieved by decreasing the inlet pressure to the lower column turbine. If a separate compressor is used, this reduction in turbine inlet pressure is achieved by adjusting the outlet pressure of the machine by utilizing either guidevanes or a suction throttle valve. This allows for a decrease in liquid product with an associated decrease in power, albeit at a slight cost penalty. The disadvantage to this alternative is that it is capital intensive in that it requires a separate compressor including motor, skid, lube oil system, etc. This is in addition to the same components being required for both the product boiler compressor and turbine.
The turbine loaded booster is a less expensive alternative, however there is no power savings associated with liquid turndown. Reducing the inlet pressure to the compressor will result in a lower outlet pressure and reduced liquid. However, since the booster is loaded by the turbine, there is no electrical power reduction. Power savings could be achieved by lowering the inlet pressure to the booster via a reduction in the main air compressor discharge pressure. However, the discharge pressure of the main air compressor must remain constant for the product boiler compressor to be able to meet its requirement. Therefore, there are no power savings available with using a turbine loaded booster compressor for liquid production.
Another problem with conventional systems is the selection of the product boiler compressor itself. The product boiler compressor is used to elevate the air pressure to that level needed to boil the liquid oxygen in the product boiler. As discussed above with relation to the turbine booster, a separate compressor for this is cost prohibitive. To reduce costs, extra pinions may be added to the main air compressor, which allows the addition of one or more stages of product boiler compression onto the main air compressor. The disadvantage of this alternative is the difficulty in achieving good efficiencies from these product boiler wheels. This is because the speed of the bullgear is set to optimize the efficiency of the main compressor wheels, and this is typically not the best speed for the product boiler wheels.
In summary, the problem is that there is presently no system that allows varying of the liquid production, at constant product gaseous oxygen pressure, in a cost effective and efficient manner. For plants that are designed for liquid products above some minimal quantity, turndown of liquid production is very important. Not being able to reduce the liquid production detracts from the ability of the plant to respond to changing market conditions. When a plant is built, there may not be an immediate demand for large quantities of liquid. However, if the market demand increases, having a plant that can produce large quantities of liquid but can produce lesser quantities efficiently would be of high value.
Accordingly, it is an object of this invention to provide a cryogenic air separation system which can efficiently produce gaseous product, particularly at a defined elevated pressure, and also liquid product wherein the liquid production may change.